This invention relates to switch control circuits and, more particularly, to overcurrent trip circuits used to control a power switch.
Electrical power systems, such as single or multiple phase AC or DC voltage types are required to be operative over a range of conditions limited by the current handling capacity of the wiring and loads. Due to cost and size considerations, it is frequently desirable to minimize the power handling capability of the system. However, for safety reasons, it is required to provide devices for the system to determine when current levels approach an overload condition, i.e., the point at which wiring and/or components may be damaged. As an example, in aircraft electrical power systems, the need for minimum size and weight, accompanied by high reliability and safety, is particularly acute. Conventional methods of overcurrent protection typically sense the instantaneous current and open the circuit if the current exceeds a trip threshold. In general, the time required by the switch to open the circuit after detection of the overcurrent may allow the current to reach damaging levels. This is particularly true in the case of resonant power supply currents which are designed to operate with high current pulses and a lowe inductance source.
When a load is powered by a low impedance source, a power control switch is used not only to control the power to the load but also to protect the system from abnormal loadings and short circuits. If a short circuit occurs at the load, the current immediately rises. The rate of rise is dependent on the source voltage and line impedance. Once the current reaches a preselected over-current trip point, a control circuit initiates a signal which opens the switch. Since the detector circuit and switch take a finite time to operate, the current continues to rise above the trip point. For example, in a 270 volt DC system with a line impedance of one microhenry and a switch opening time of 1 microsecond, the actual current will reach 270 amps above the trip threshold level. It is therefore desirable to devise a control system which opens the circuit before the current is permitted to reach damaging levels.